Signature mark recognition systems

ABSTRACT

A signature recognition system for identifying an article with a distinctive diffraction element (or elements) and verifying the presence of that element or elements comprising: an article with one or more diffraction gratings impressed thereon, the grating(s) exhibiting periodic wave surface profiles having a depth-to-pitch ratio δ of between 0.1 and 0.5, a source of polarized electromagnetic radiation of wavelength λ such that the pitch G of the periodic wave surface profile of the grating(s) is comparable to an integer multiple n of that wavelength, a device for directing the source of polarized electromagnetic radiation to the surface of the grating(s) at a plane of incidence substantially normal to the plane of the surface of the diffraction grating and at an angle of approximately 45° azimuth to the alignment of the grooves on the surface, and a device for detecting radiation reflected from the grating(s) surface which is oppositely polarized to the incident radiation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to signature recognition systems for providingarticles with distinctive signatures and means for verifying thosesignatures.

2. Discussion of Prior Art

Patent Application no. GB 2235287 B discloses an optical sensor based onthe use of surface plasmon polaritons (SPP). The sensor comprisesapparatus for detecting a surface plasmon-polariton resonance maximumwhich occurs following polarisation conversion of particular wavelengthsof radiation incident upon a surface which correspond to the excitationof an SPP at or about its resonant frequency.

Bar code systems are well known as a means of distinguishing certainitems and are easily read using light pens. As a two dimensional system,bar codes are easily distorted by smudges of dirt, creases, scratchesand so on, this can cause errors in readings taken by a light pen.Furthermore, as they are visible to the naked eye, conventional bar codesystems are fairly simple to copy or alter.

Magnetic strips and reading devices are also commonly used as a securitymeasure for identifying personal identification cards, credit cards andthe like. Like conventional optical bar codes, these strips are easilydamaged by bending or scratching and can also be affected by closecontact with other magnetic sources.

SUMMARY OF THE INVENTION

The present invention is a signature recognition system for identifyingan article with a distinctive diffractive element (or elements) andverifying the presence of that element or elements comprising;

an article with one or more diffraction gratings impressed thereon, thegrating(s) exhibiting periodic wave surface profile having adepth-to-pitch ratio δ of between 0.1 and 0.5,

a source of polarised electromagnetic radiation of wavelength λ suchthat the pitch G of the periodic wave surface profile of the grating(s)is comparable to an integer multiple n of that wavelength

means for directing the source of polarised electromagnetic radiation tothe surface of the grating(s) at a plane of incidence substantiallynormal to the plane of the surface of the diffraction grating and at anangle of approximately 45° azimuth to the alignment of the grooves onthe surface of the diffraction grating, and

means for detecting radiation reflected from the grating(s) surfacewhich is oppositely polarised to the incident radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the reflection of radiation whichis oppositely polarised to the incident radiation;

FIG. 2 illustrates graphs of reflectivity versus wavelength fordifferent pitch-to-depth ratios;

FIG. 3 is a graph of reflectivity versus wavelength for various incidentangles;

FIG. 4 is a schematic illustrating one embodiment of the presentinvention;

FIGS. 5a and 5 b are top and side views, respectively, of one embodimentof the present invention; and

FIG. 6 is a top view of a further embodiment of the present invention.

DETAILED DISCUSSION OF EMBODIMENTS

It can be shown that when polarised electromagnetic radiation isdirected to a suitably proportioned diffraction grating under theconditions described, the reflected radiation is oppositely polarised tothe incident radiation. A schematic of these conditions is illustratedin FIG. 1 wherein a source of radiation (1) is made incident upon agrating (2) with grooves (3) aligned at azimuthal angle (4) to the planeof incidence (5). When the plane of incidence (5) is substantiallynormal to the grating surface (2), radiation of opposite polarisation(6) is reflected back along the plane of incidence (5).

The phenomenon is defined as polarisation conversion. Unlike GB 2235287B the effect is dependent on diffractive surfaces that alter thepolarisation state of incident radiation. This effect is due to thegeometry of the surface, and can be exhibited by any suitably-profiledreflective material, the frequency range of operation being dictated bythe dimensions of that profile. As the effect is dependent on a closerelationship between the geometric surface profile of the grating andthe wavelength of radiation incident upon it, detection of an oppositelypolarised wavelength of radiation reflected from a grating or series ofgratings is indicative of specific surface profile dimensions of agrating. Suitable such profiles include sinusoidal, square andtriangular waves. Most preferred is the sine wave profile as this islikely to provide the greatest amount of polarisation conversion of thesource with minimal dispersion effects.

The strongest polarisation-conversion effects can be obtained from agrooved reflective surface under the following conditions:

The grooves are aligned at 45 degrees to the plane of incidence (i.e.the azimuthal angle is 45 degrees).

The radiation is substantially normally incident upon the surface (i.e.the angle of incidence is said to be approximately zero).

The wavelength λ of the incident radiation is given by the expression:

G/n=λ

in which n is an integer and G is the pitch of the surface, i.e. therepeat period or in the specific case of a sinusoidal surface profile,the peak-to-peak separation.

The most efficient polarisation conversion effect occurs when n=1. FIG.2 shows a plot of reflectivity versus wavelength for variouspitch-to-depth ratios under the conditions described. As can be seen,the relationship between the depth-to-pitch ratio δ and the range ofwavelengths which may undergo polarisation conversion can be broadlycategorised as follows;

When the depth-to-pitch ratio δ (δ=d/G) is between ˜0.1 and ˜0.3, thepolarisation-conversion is exhibited in a plot of reflectivity versuswavelength as a distinct peak.

When the depth-to-pitch ratio δ (δ=d/G) exceeds ˜0.3, the peak broadensto longer wavelengths, producing a plateau in a plot of reflectivityversus wavelength.

In the former case the grating surfaces will exhibit a peak value ofreflectivity, sufficient to enable a polychromatic reading device todistinguish between different diffractive elements. Such a gratingsurface will be useful where a very high degree of distinguishability isnecessary between similar signatures.

FIG. 3 shows a plot of reflectivity versus wavelength for variousincident angles under the conditions described. As can be seen from theFigure, as the angle of incidence is increased, the peak splits into twoseparate maxima that move to higher and lower wavelengths respectivelyas the angle increases. The peaks also decrease in efficiency as theangle of incidence increases. This effect will enable the utilisation ofnon-zero angles of incidence up to about 30 degrees.

In the latter of the above cases where the depth-to-pitch ratio δ(δ=d/G) is between ˜0.3 and ˜0.5, a broader spectrum of wavelengths willbe polarisation-converted by the grating surface, a feature that theskilled person will understand to be of use where the exact wavelengthof the radiation source is poorly defined, or the intensity of thereflected signal needs to be increased by accessing a range ofwavelengths from a broad-band source. A system employing such a gratingwould be useful where a larger margin of error must be allowed for, forinstance in coding foodstuffs for transmission through supermarketcheckouts where signatures need to be identified quickly and thediffractive grating cannot always be positioned accurately in relationto the radiation source.

One convenient method of directing the source of electromagneticradiation to the surface of the grating(s) in accordance with theinvention is to use a circularly polarised source of the radiation. FIG.4 illustrates such a system.

In FIG. 4, electromagnetic radiation from source (1) is positioned todirect the source in a direction substantially normal to the diffractiongrating surface (2). The source-radiation first passes through a linearpolariser 43, and then through a 90° phase-retardation plate 44, thecombination of 43 and 44 acting as a circular polariser. The source thenarrives at the diffraction grating surface (2) on the article underdetection. Any part of the circularly polarised source which is incidentto the grating at 45° azimuth will undergo polarisation conversion: thereflected beam can then be transmitted back through the circularpolariser. If polarisation conversion did not occur (i.e. if thecorrectly-profiled grating was absent) then the reflected radiationwould be rotating in a sense that would be opposed to that of thepolariser, and transmission could not occur. The reflected radiationwill therefore only produce a signal at the detector 45 if the surfaceexhibits specifically-tailored diffractive properties.

In one particular embodiment of the invention a series of gratings areimpressed on a card, for instance, a credit card or securityidentification card. The gratings may be of the same profile and spacedapart or may be of the same orientation but with surface profiles ofdifferent dimensions. Thus various combinations of gratings can produceunique identification codes for users of personal credit or securitycards.

In the simplest case, a monochromatic light source is polarised andplaced above an appropriate grating or series of gratings. A suitablelight detector is covered with an oppositely-aligned polariser. Theradiation emitted from the source will then be reflected from thegrating surface at near-normal incidence, and a signal will be detectedonly if polarisation conversion has occurred. Thus a binary code can beprovided with gratings causing intermittent polarisation conversionalong a series of gratings. A further level of differentiation betweencodes can be provided by varying the widths of a series of similargratings providing an effect much like that of conventional optical barcodes. Optionally a conventional optical bar code could be imprintedonto a continuous diffraction grating to provide this effect. In thelatter two cases, existing bar code reading equipment could be readilymodified to read the codes of the present invention by placing opposingpolarisers over the existing light sources and detectors.

The polarisation conversion effect is so surface specific that mostsurfaces will not produce any signal at all (and almost certainly not ofthe correct wavelength in the case of a polychromatic source ofradiation) and hence small damaged areas of a grating will merely reducethe total magnitude of the signal detected rather than produce spurioussignals, thus the scope for error in readings is much reduced overconventional systems.

If a polychromatic radiation source is used then the wavelengthproducing the most intense polarisation converted signal could bedetected. It follows from this that a series of gratings designed toproduce the effect at different wavelengths could be distinguished. Byvarying the arrangement of gratings of differing wavelength polarisationconversion characteristics, individual cards can be given uniqueidentification codes. Again the gratings could be spaced apart and/or ofvarying lengths to provide a further discriminating feature in the code.

An alternative embodiment is shown in FIGS. 5a and 5 b. A pattern ofgratings A and B according to the present invention are provided alongtracks to be followed by, for instance, a robot vehicle C. The robotcould be programmed to follow a particular pattern or to turn or stop onrecognising other patterns.

As the gratings are necessarily three dimensional and their dimensionsare in the sub-nanometric range, they become very difficult to copy oralter. To prevent reduction in signal magnitudes resulting from dirty orscratched grating surfaces, the gratings could be coated with dielectricmaterials.

A further degree of resolution can be obtained by placing two detectiondevices in parallel, one detecting polarisation converted reflections,the other detecting remaining reflections. A comparison of the twodetected signals provides a higher resolution measurement of thepolarisation converted radiation.

Whilst it is envisaged that the use of optical or infrared componentrywould be most convenient for the embodiments so far described (primarilydue to the size of the equipment required), an alternative embodimentuses larger gratings and higher wavelength radiation such as microwaves.As the effect is angle specific as well as surface geometry dependent,the device lends itself to use as a micro-positioning device. Signalsgenerated by moving devices are detected only when the devices are nearparallel to the grating. For instance, as shown in FIG. 6, this effectcould be used in the design of automatic radar for keeping road vehiclesD in lanes via road side barriers with gratings E which detect when thevehicles are within their range.

What is claimed is:
 1. A signature recognition system for identifying anarticle with at least one distinctive diffractive element and verifyingthe presence of said element comprising: an article with at least onediffraction grating impressed thereon, the grating exhibiting periodicwave surface profiles having a depth-to-pitch ratio δ of between 0.1 and0.5, a source of polarised electromagnetic radiation of wavelength λsuch that the pitch G of the periodic wave surface profile of thegrating is comparable to an integer multiple n of that wavelength, meansfor directing the source of polarised electromagnetic radiation to thesurface of the grating at a plane of incidence substantially normal tothe plane of the surface of the diffraction grating and at an angle ofapproximately 45° azimuth to the alignment of the grooves on thesurface, and means for detecting radiation reflected from the gratingsurface which is oppositely polarised to the incident radiation.
 2. Asignature recognition system as claimed in claim 1 wherein the source ofpolarised electromagnetic radiation is circularly polarised.
 3. Asignature recognition system as claimed in claim 1 wherein the source ofpolarised radiation is plane polarised.
 4. A signature recognitionsystem as claimed in claim 1 wherein a series of gratings with surfaceprofiles of similar dimension are spaced apart at intervals to form anidentifiable pattern.
 5. A signature recognition system as claimed inclaim 4 wherein the gratings are of differing width.
 6. A signaturerecognition system as claimed in claim 4 wherein the source ofelectromagnetic radiation is light.
 7. A signature recognition system asclaimed in claim 1 wherein a series of gratings with surface profiles ofdiffering dimensions are impressed on the article and the source ofelectromagnetic radiation is polychromatic.
 8. A signature recognitionsystem as claimed in claim 7 wherein the gratings are of differinglength.
 9. A signature recognition system as claimed in claim 1 whereinthe article is a credit card or security identification card.
 10. Asignature recognition system as claimed in claim 1 wherein the wavesurface profile is a sine wave.
 11. A signature recognition system asclaimed in claim 1 wherein the grating surface is coated with adielectric material.
 12. A signature recognition system as claimed inclaim 1 wherein the grating surface has a depth to pitch ratio ofbetween 0.1 and 0.3.
 13. A signature recognition system as claimed inclaim 1 wherein the grating surface has a depth to pitch ratio ofbetween 0.3 and 0.5.
 14. A signature recognition system for identifyingan article with a distinctive diffractive element (or elements) andverifying the presence of that element or elements comprising: anarticle with one or more diffraction gratings impressed thereon, thegrating(s) exhibiting periodic wave surface profiles having adepth-to-pitch ratio δ of between 0.1 and 0.5, a source of polarisedelectromagnetic radiation of wavelength λ such that the pitch G of theperiodic wave surface profile of the grating(s) is comparable to aninteger multiple n of that wavelength, means for directing the source ofpolarised electromagnetic radiation to the surface of the grating(s) ata plane of incidence substantially normal to the plane of the surface ofthe diffraction grating and at an angle of approximately 45° azimuth tothe alignment of the grooves on the surface, and means for detectingradiation reflected from the grating(s) surface which is oppositelypolarised to the incident radiation, wherein a series of gratings withsurface profiles of differing dimensions are impressed on the articleand the source of electromagnetic radiation is polychromatic, whereinthe gratings are spaced apart at intervals to form an identifiablepattern.
 15. A signature recognition system for identifying an articlewith a distinctive diffractive element (or elements) and verifying thepresence of that element or elements comprising: an article with one ormore diffraction gratings impressed thereon, the grating(s) exhibitingperiodic wave surface profiles having a depth-to-pitch ratio δ ofbetween 0.1 and 0.5, a source of polarised electromagnetic radiation ofwavelength λ such that the pitch G of the periodic wave surface profileof the grating(s) is comparable to an integer multiple n of thatwavelength, means for directing the source of polarised electromagneticradiation to the surface of the grating(s) at a plane of incidencesubstantially normal to the plane of the surface of the diffractiongrating and at an angle of approximately 45° azimuth to the alignment ofthe grooves on the surface, and means for detecting radiation reflectedfrom the grating(s) surface which is oppositely polarised to theincident radiation, wherein the article comprises a track and thedetector comprises a robotic vehicle programmed to follow the track. 16.A signature recognition system for identifying an article with adistinctive diffractive element (or elements) and verifying the presenceof that element or elements comprising: an article with one or morediffraction gratings impressed thereon, the grating(s) exhibitingperiodic wave surface profiles having a depth-to-pitch ratio δ ofbetween 0.1 and 0.5, a source of polarised electromagnetic radiation ofwavelength λ such that the pitch G of the periodic wave surface profileof the grating(s) is comparable to an integer multiple n of thatwavelength, means for directing the source of polarised electromagneticradiation to the surface of the grating(s) at a plane of incidencesubstantially normal to the plane of the surface of the diffractiongrating and at an angle of approximately 45° azimuth to the alignment ofthe grooves on the surface, and means for detecting radiation reflectedfrom the grating(s) surface which is oppositely polarised to theincident radiation, wherein the article comprises road side barrierdevices and the detector comprises a road vehicle.
 17. A signaturerecognition system as claimed in claim 16 wherein the source ofelectromagnetic radiation is in the microwave range.